The present invention relates to chemical methods for scavenging hydrogen sulfide (H2S) from molten sulfur, and more particularly relates, in one embodiment, to methods of inhibiting the evolution of H2S from molten sulfur.
The removal of H2S from various streams is a problem that has challenged many workers in many industries. One such industry concerns streams and quantities of molten sulfur. When sulfur is produced in a refinery it is in a molten or liquid form that is typically stored in pits in the ground or possibly in insulated storage tanks. The reaction of the sulfur with hydrocarbon impurities present in the material and the decomposition of sulfhydryl compoundsxe2x80x94typically with the general formula Hxe2x80x94Sxe2x80x94(S)xxe2x80x94Sxe2x80x94Hxe2x80x94 which are also present in the sulfur form hydrogen sulfide. The hydrogen sulfide in turn is a safety and odor problem. The problem may occur at the refinery in their storage pits/tanks or in vessels such as rail cars and tank trucks, which transport the sulfur from the refinery. It is desirable to prevent the evolution of hydrogen sulfide from molten sulfur during storage and/or distribution.
The presence of H2S presents many environmental and safety hazards. Hydrogen sulfide is highly flammable, toxic when inhaled, and strongly irritates the eyes and other mucous membranes. Flaring of gas that contains H2S does not solve the problem for gas streams because, unless the H2S is removed prior to flaring, the combustion products will contain unacceptable amounts of pollutants, such as sulfur dioxide (SO2)xe2x80x94 a component of xe2x80x9cacid rain.xe2x80x9d
Hydrogen sulfide has an offensive odor, and natural gas containing H2S often is called xe2x80x9csourxe2x80x9d gas. Treatments to reduce or remove H2S from substrates often are called xe2x80x9csweeteningxe2x80x9d treatments. The agent that is used to remove or reduce H2S levels sometimes is called a xe2x80x9cscavengingxe2x80x9d agent. The prevention of H2S evolution from molten sulfur is only one example of where H2S level inhibition, reduction or removal must be performed.
The problem of removing or reducing H2S from molten sulfur has been solved in many different ways in the past. Oxidizers such as sodium or calcium hypochlorite or hypobromite have been used as scavengers in liquid sulfur. Air has also been used as an oxidizer to convert H2S to elemental sulfur.
Other approaches involve intentionally promoting the evolution or degassing of H2S from the molten sulfur. That is, various additives are used to intentionally cause sulfhydryl species such as Hxe2x80x94Sxe2x80x94(S)xxe2x80x94Sxe2x80x94H to decompose. The H2S formed is then swept away to a vapor recovery system. Once these materials are decomposed, the sulfur is left with reduced potential to form more H2S and may be relatively safer to transport. Typical compounds used in such methods contain nitrogen.
U.S. Pat. No. 5,552,060 describes a method for scavenging H2S from aqueous and hydrocarbon substrates using an epoxide. Preferred epoxides are styrene oxide, 1,3-butadiene diepoxide, and cyclohexene oxide.
A continuing need exists for alternative processes and compositions to inhibit H2S evolving from molten sulfur. It would be desirable if compositions and methods could be devised to aid and improve the ability accomplish this task and do not any disadvantageous impact on the end uses of the sulfur.
Accordingly, it is an object of the present invention to provide a method for inhibiting the evolution of H2S from the molten sulfur.
It is another object of the present invention to inhibit the evolution of H2S from the molten sulfur by adding a readily available scavenging agent to the molten sulfur.
Another object of the present invention is to inhibit the evolution of H2S from the molten sulfur without adversely affecting the quality of the sulfur.
In carrying out these and other objects of the invention, there is provided, in one form, a method for inhibiting the evolution of hydrogen sulfide (H2S) from molten sulfur involving contacting the molten sulfur contaminated with at least one sulfhydryl compound with an effective amount of a scavenging agent which include anhydrides, conjugated ketones, carbonates, epoxides, monoesters and diesters of unsaturated dicarboxylic acids and/or polymers of these esters, where the scavenging agent is a liquid at ambient temperature.
It has been discovered that the scavenging agents of the present invention may be used to treat molten sulfur that contain xe2x80x9csulfhydryl compounds,xe2x80x9d such as hydrogen sulfide (H2S), organosulfur compounds having at least one sulfhydryl (xe2x80x94SH) group, known as mercaptans, also known as thiols (Rxe2x80x94SH, where R is a hydrocarbon group), thiol carboxylic acids (RCOxe2x80x94SH), dithio acids (RCSxe2x80x94SH), and related compounds. Sulfhydryl compounds may also be represented by the very general formula Hxe2x80x94Sxe2x80x94(S)xxe2x80x94Sxe2x80x94H.
It will be appreciated that by the term H2S evolution inhibition is meant any prevention, suppression, hindrance, impeding, controlling, lowering, diminishing, retarding, abatement, decreasing or other reduction in the amount of H2S evolved as compared with the case where a scavenging agent of this invention is not employed. Further, it should be understood that the inventive method is a success as long as H2S evolution is reduced or inhibited at least somewhat from the levels that would occur in the absence of the scavenging agent. It is not necessary that H2S evolution completely cease for the method of the invention to be considered successful.
The scavenging agents of the present invention may be monoesters and diesters of, anhydrides, conjugated ketones, carbonates, epoxides, and/or unsaturated dicarboxylic acids.
In the case of the epoxides, and without wanting to limit the invention to any particular theory, the epoxide portion of the molecule is believed to react with the sulfhydryl compounds according to the following equation: 
Any epoxide should function in the present invention as long as the remainder of the compound does not interfere with this reaction.
Epoxides suitable for use in the present invention generally have the formula: 
where R1, R2, R3, and R4 independently are selected from the group consisting of hydrogen and hydrocarbon groups having between about 1-20 carbon atoms, selected from the group consisting of straight, branched, and cyclic alkyl groups, aryl, alkaryl, and aralkyl groups, and straight, branched, and cyclic alkyl groups substituted with oxygen, heterocyclic alkyls containing oxygen as a ring constituent, wherein R2 and R3 may be joined to form a cycloalkyl or a heterocyclic alkyl having oxygen as a ring constituent.
Preferred scavenging agents are those that are liquid at ambient temperature. Thus, in the case of epoxides, ethylene oxide, which is a gas at ambient temperature, would be excluded in this embodiment. Particularly preferred epoxide scavenging agents include, but are not necessarily limited to, styrene oxide, ethylhexyl glycidyl ether, butyl glycidyl ether, butylene oxide, 1-decene oxide, phenyl glycidyl ether, epoxidized fatty acids and esters, and mixtures thereof.
As noted, the epoxide portion of the molecule is believed to be the functional group that actually reacts with the sulfur moiety; therefore, molecules in which the epoxide group is more xe2x80x9caccessiblexe2x80x9d to the sulfur should be more efficient scavengers.
Epoxides suitable for use in the present invention are commercially available from a number of sources. Epoxides also may be readily prepared using well established procedures, such as those described in Morrison and Boyd, Organic Chemistry (5th Ed. 1987) pp. 713-715, incorporated herein by reference.
Monoesters and diesters of unsaturated dicarboxylic acids having the formula 
have also been found to be useful in inhibiting H2S evolution, where R5 are independently selected from the group consisting of hydrogen, C1 to C12 alkyl, alkenyl, aryl and polyhydric alcohol moieties having 1 to 60 carbon atoms, preferably 1-30 carbon atoms. In one non-limiting embodiment of the invention, the ethylene glycol ester of maleic acid is a preferred scavenging agent. Polymers of these esters are also expected to be useful scavenging agents in this invention. Polymerization is expected to take place through the unsaturation in the backbone or through the R5 of the ester group or the ester group itself. Non-limiting examples of polymers through the R5 of the ester group may be illustrated as follows: 
where R5xe2x80x2 may independently be selected from the group consisting of ethylene, straight or branched propylene and straight or branched butylene groups. In one non-limiting embodiment of the invention, it is preferred that the polymers contain unsaturation. Without wishing to be limited to any particular mechanism, the scavenging activity may be due to the unsaturation.
Anhydrides suitable as scavenging agents in the method of this invention include, but are not necessarily limited to, maleic anhydride, phthalic anhydride and those having the formula: 
where R6 is selected from the group consisting of hydrogen, C1 to C12 alkyl, aryl, and alkenyl and polyhydric alcohol moieties having 1 to 12 carbon atoms. Non-limiting examples of suitable anhydrides include dodecenylsuccinic anhydride and succinic anhydride, maleic anhydride, dodecyl succinic anhydride, polybutenyl succinic anhydride, and mixtures thereof. It is expected that polymers of these anhydrides (e.g. polymerized through the R6 group) would be useful as scavenging agents in this invention.
Conjugated ketones are also useful scavenging agents to inhibit H2S in the method of this invention. Suitable conjugated ketones include, but are not necessarily limited to those of the formula: 
where R6 are independently selected from the group consisting of hydrogen, C1 to C12 alkyl, aryl, and alkenyl. Non-limiting examples of suitable conjugated ketone include, but are not necessarily limited to, 2,4-pentadione; 2,4-pentadione, and the like.
Suitable carbonates include linear and cyclic carbonates having the formula: 
where R7 is independently selected from the group consisting of hydrogen, C1 to C12 straight and branched alkyl, aryl, alkenyl, cyclic and non-cyclic alkyl, aryl, or alkenyl. Non-limiting examples of suitable carbonate for the method of this invention include, but are not necessarily limited to, propylene carbonate, ethylene carbonate, diethylene carbonate.
The temperature at which the scavenging agent is contacted with the liquid sulfur may be between about 38 and 232xc2x0 C. (about 100 and about 450xc2x0 F.), preferably between about 121 and about 177xc2x0 C. (about 250 and about 350xc2x0 F.).
As noted, the scavenging agent preferably may be added to the molten sulfur at temperatures such that the material is at least flowable for ease in mixing and transporting to the storage vessel or transport vehicle. The scavenging agent should, in one embodiment, be completely mixed with the molten sulfur so that the sulfhydryl compounds may be effectively contacted, reacted and scavenged. The mixing operation itself is not believed to be critical as long as the stated purpose of the invention is accomplished. The mixing may be performed by any known technique, including but not limited to a static mixer, a paddle or impeller or mixing techniques yet to be developed.
In another embodiment, the scavenging additive itself or the scavenging agent in solution is atomized into the vapor phase or vapor space of the vessel above the molten sulfur. Atomizing within the context of this invention includes dividing the liquid into a fine spray, mist or cloud of particles or droplets. Sufficient mixing may occur at the surface of the molten sulfur the atomized liquid, cloud or mist. Stirring or agitation of the molten sulfur would increase contact and the rate of contact with the scavenging agent. Generally, the smaller the size of the droplets or particles, the better the contact with the molten sulfur.
Suitable solvents for solutions of the scavenging agents of the invention herein include, but are not necessarily limited to, water, alcohols, glycol, and other organic solvents, depending upon the solubility of the particular scavenging agent in the particular solvent. The amount of solvent should be limited to the minimum amount necessary to place the scavenger in an easy-to-handle, liquid form. A concentrated product minimizes the amount of contamination of the sulfur possibly caused by the product.
The molar amount of scavenging agent to sulfhydryl compound ranges from about 0.01 to 1 to about 100 to 1, in one non-limiting embodiment. Preferably this molar ratio ranges from about 0.5 to 1 to about 1.5 to 1. In another non-limiting embodiment of the invention, where the molten sulfur comprises a molar amount of at least one sulfhydryl compound, the effective molar amount of the scavenging agent is at least substantially equal to the molar amount of the sulfhydryl compound. In another embodiment, the effective molar amount of the epoxide is substantially equal to the molar amount of the sulfhydryl compounds present. In one non-limiting embodiment, the molten sulfur should be treated with between about 100 to about 5,000 ppm, preferably between about 500 to about 2,000 ppm of scavenging agent.
The molten sulfur should be treated with the scavenging agent until reaction with hydrogen sulfide and/or with other sulfhydryl compounds, has produced a product in which the sulfhydryls in the molten sulfur have been removed, scavenged, reacted or otherwise inhibited from evolving H2S. The amount of scavenging agent added should be sufficient to reduce the H2S over the molten sulfur to a level low enough that concentrations at tank hatches, vents, etc. do not exceed safety limits or cause odor problems. It is noted that the current OSHA exposure limits to H2S or 15 ppm maximum for a 15 minute exposure (STEL, short term exposure limit) and 10 ppm time weighted average over an 8 hour period. In one non-limiting embodiment of the invention, the amount of scavenging agent contacted with the molten sulfur is sufficient to keep the amount of the H2S evolved in a vapor phase over the molten sulfur to 2000 ppm or less, preferably 100 ppm or less, and most preferably 10 ppm or less.
It will be appreciated that the method of this invention does not have an undesirable affect on the sulfur treated. In particular, the method and scavenging agent of this invention do not discolor the sulfur.